(1) Field of the Invention
The present invention relates to a light control element, in particular, to a light control element that equips a base plate having an electrooptic effect, with an optical guide and an electrode for modulation, and where ridge structure is formed on said base plate.
(2) Related Art Statement
In the technical fields of optical communication, optical measurement, etc., light control elements such as an optical switch, an optical phase modulator, and an optical intensity modulator are used frequently.
The light control element that can operate with low drive voltage at high speed is useful. Particularly, the light control element where an optical guide and an electrode for modulation are formed on a base plate having an electrooptic effect, such as LiNbO3 (hereinafter described as LN) has been attracting attention.
For the light control element like this, it is necessary that the electric field generated by the electrode for modulation is impressed effectively to the optical guide, and for this purpose, the optical guide is formed on the ridge part or between the ridge on the base plate as in Japanese Patent No. 2550606 and Japanese Provisional Publication No. 2001-350050.
However, for the light control element using the base plate that has an electro optic effect, such as a LN optical modulator, the bias point of the modulation voltage that is impressed to said light control element deviates from the appropriate value gradually due to the direct current voltage impressed to said light control element, temperature change, and temporal change. This is called DC drift phenomenon. Particularly when an optical modulation reaches as much as 40 GHz, resulting from the progress of the broadband and high-speed optical communication in recent years, huge DC drift phenomenon occurs.
Further, the light control element with ridge structure tends to have the DC drift phenomenon, and it is essential to control this DC drift phenomenon for the purpose of drive stability of the light control element.
As a method for controlling DC drift phenomenon, Japanese Kokoku Publication No. 1993-78016 discloses the method of installing a conducting layer between electrodes. This method prevents the operation point of drive voltage from changing since the polarization inside a base plate changes due to the temperature change of the base plate.
Also, the prior application, Japanese Patent Application No. 2000-399927 (Provisional Publication No. 2002-202483), filed by the present applicant proposes the method of annealing a base plate in an oxygen-containing ambient atmosphere during the production process. This is to prevent DC drift phenomenon from occurring due to lack of oxygen inside the base plate resulting from overetching of a buffer layer.
However, the above-described methods for controlling the DC drift phenomenon cannot sufficiently control the DC drift phenomenon of the light control element having ridge structure, and particularly when the light control element is used in a large area, it has been a major problem.
The present invention intends to solve the above-mentioned problems, to control DC drift phenomenon of the light control element having ridge structure, and to provide the light control element which has high drive stability in a large area.